The objective of this study was to investigate the relationship of the nutrient content of duck-weed biomass to duck-weed growth in swine wastewater. Batch tests of Spirodela punctata 7776, the selected strain for highest total protein production, were conducted in an environment-controlled growth chamber at 24 degrees C and 16 h of light per day. A prolonged growth period was observed after the nutrients in the medium were exhausted, indicating that duckweed could use its stored nutrients for growth. Brediction of growth using medium concentration as an independent variable was deemed unsuitable to describe this growth. Throughout the 30-day growing period, nitrogen and phosphorus content in the biomass varied from 59.7 to 19.7 mg(N)/g(biomass) and from 14.8 to 6 8 mg(P)/g(biomass) (dry weight basis), respectively. The relationship between biomass nitrogen content and specific growth rate of Spirodela punctata 7776 was found to follow Monod-type kinetics with mu(max) of 0.24 g(N)/g(biomass)/day and K-N of 28.8 mg(P)/g(biomass). Reduced growth rate was observed in the duckweed culture with high duckweed density (mass per unit area). Effects of the duckweed density on growth rate and nutrient uptake are modeled and discussed.